1//! Synchronization objects that employ poisoning.
2//!
3//! # Poisoning
4//!
5//! All synchronization objects in this module implement a strategy called "poisoning"
6//! where if a thread panics while holding the exclusive access granted by the primitive,
7//! the state of the primitive is set to "poisoned".
8//! This information is then propagated to all other threads
9//! to signify that the data protected by this primitive is likely tainted
10//! (some invariant is not being upheld).
11//!
12//! The specifics of how this "poisoned" state affects other threads
13//! depend on the primitive. See [#Overview] bellow.
14//!
15//! For the alternative implementations that do not employ poisoning,
16//! see `std::sys::nonpoisoning`.
17//!
18//! # Overview
19//!
20//! Below is a list of synchronization objects provided by this module
21//! with a high-level overview for each object and a description
22//! of how it employs "poisoning".
23//!
24//! - [`Condvar`]: Condition Variable, providing the ability to block
25//! a thread while waiting for an event to occur.
26//!
27//! Condition variables are typically associated with
28//! a boolean predicate (a condition) and a mutex.
29//! This implementation is associated with [`poison::Mutex`](Mutex),
30//! which employs poisoning.
31//! For this reason, [`Condvar::wait()`] will return a [`LockResult`],
32//! just like [`poison::Mutex::lock()`](Mutex::lock) does.
33//!
34//! - [`Mutex`]: Mutual Exclusion mechanism, which ensures that at
35//! most one thread at a time is able to access some data.
36//!
37//! [`Mutex::lock()`] returns a [`LockResult`],
38//! providing a way to deal with the poisoned state.
39//! See [`Mutex`'s documentation](Mutex#poisoning) for more.
40//!
41//! - [`Once`]: A thread-safe way to run a piece of code only once.
42//! Mostly useful for implementing one-time global initialization.
43//!
44//! [`Once`] is poisoned if the piece of code passed to
45//! [`Once::call_once()`] or [`Once::call_once_force()`] panics.
46//! When in poisoned state, subsequent calls to [`Once::call_once()`] will panic too.
47//! [`Once::call_once_force()`] can be used to clear the poisoned state.
48//!
49//! - [`RwLock`]: Provides a mutual exclusion mechanism which allows
50//! multiple readers at the same time, while allowing only one
51//! writer at a time. In some cases, this can be more efficient than
52//! a mutex.
53//!
54//! This implementation, like [`Mutex`], will become poisoned on a panic.
55//! Note, however, that an `RwLock` may only be poisoned if a panic occurs
56//! while it is locked exclusively (write mode). If a panic occurs in any reader,
57//! then the lock will not be poisoned.
58
59// FIXME(sync_nonpoison) add links to sync::nonpoison to the doc comment above.
60
61#[stable(feature = "rust1", since = "1.0.0")]
62pub use self::condvar::{Condvar, WaitTimeoutResult};
63#[unstable(feature = "mapped_lock_guards", issue = "117108")]
64pub use self::mutex::MappedMutexGuard;
65#[stable(feature = "rust1", since = "1.0.0")]
66pub use self::mutex::{Mutex, MutexGuard};
67#[stable(feature = "rust1", since = "1.0.0")]
68#[expect(deprecated)]
69pub use self::once::ONCE_INIT;
70#[stable(feature = "rust1", since = "1.0.0")]
71pub use self::once::{Once, OnceState};
72#[unstable(feature = "mapped_lock_guards", issue = "117108")]
73pub use self::rwlock::{MappedRwLockReadGuard, MappedRwLockWriteGuard};
74#[stable(feature = "rust1", since = "1.0.0")]
75pub use self::rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};
76use crate::error::Error;
77use crate::fmt;
78#[cfg(panic = "unwind")]
79use crate::sync::atomic::{Atomic, AtomicBool, Ordering};
80#[cfg(panic = "unwind")]
81use crate::thread;
82
83mod condvar;
84#[stable(feature = "rust1", since = "1.0.0")]
85mod mutex;
86pub(crate) mod once;
87mod rwlock;
88
89pub(crate) struct Flag {
90 #[cfg(panic = "unwind")]
91 failed: Atomic<bool>,
92}
93
94// Note that the Ordering uses to access the `failed` field of `Flag` below is
95// always `Relaxed`, and that's because this isn't actually protecting any data,
96// it's just a flag whether we've panicked or not.
97//
98// The actual location that this matters is when a mutex is **locked** which is
99// where we have external synchronization ensuring that we see memory
100// reads/writes to this flag.
101//
102// As a result, if it matters, we should see the correct value for `failed` in
103// all cases.
104
105impl Flag {
106 #[inline]
107 pub const fn new() -> Flag {
108 Flag {
109 #[cfg(panic = "unwind")]
110 failed: AtomicBool::new(false),
111 }
112 }
113
114 /// Checks the flag for an unguarded borrow, where we only care about existing poison.
115 #[inline]
116 pub fn borrow(&self) -> LockResult<()> {
117 if self.get() { Err(PoisonError::new(())) } else { Ok(()) }
118 }
119
120 /// Checks the flag for a guarded borrow, where we may also set poison when `done`.
121 #[inline]
122 pub fn guard(&self) -> LockResult<Guard> {
123 let ret = Guard {
124 #[cfg(panic = "unwind")]
125 panicking: thread::panicking(),
126 };
127 if self.get() { Err(PoisonError::new(ret)) } else { Ok(ret) }
128 }
129
130 #[inline]
131 #[cfg(panic = "unwind")]
132 pub fn done(&self, guard: &Guard) {
133 if !guard.panicking && thread::panicking() {
134 self.failed.store(true, Ordering::Relaxed);
135 }
136 }
137
138 #[inline]
139 #[cfg(not(panic = "unwind"))]
140 pub fn done(&self, _guard: &Guard) {}
141
142 #[inline]
143 #[cfg(panic = "unwind")]
144 pub fn get(&self) -> bool {
145 self.failed.load(Ordering::Relaxed)
146 }
147
148 #[inline(always)]
149 #[cfg(not(panic = "unwind"))]
150 pub fn get(&self) -> bool {
151 false
152 }
153
154 #[inline]
155 pub fn clear(&self) {
156 #[cfg(panic = "unwind")]
157 self.failed.store(false, Ordering::Relaxed)
158 }
159}
160
161#[derive(Clone)]
162pub(crate) struct Guard {
163 #[cfg(panic = "unwind")]
164 panicking: bool,
165}
166
167/// A type of error which can be returned whenever a lock is acquired.
168///
169/// Both [`Mutex`]es and [`RwLock`]s are poisoned whenever a thread fails while the lock
170/// is held. The precise semantics for when a lock is poisoned is documented on
171/// each lock. For a lock in the poisoned state, unless the state is cleared manually,
172/// all future acquisitions will return this error.
173///
174/// # Examples
175///
176/// ```
177/// use std::sync::{Arc, Mutex};
178/// use std::thread;
179///
180/// let mutex = Arc::new(Mutex::new(1));
181///
182/// // poison the mutex
183/// let c_mutex = Arc::clone(&mutex);
184/// let _ = thread::spawn(move || {
185/// let mut data = c_mutex.lock().unwrap();
186/// *data = 2;
187/// panic!();
188/// }).join();
189///
190/// match mutex.lock() {
191/// Ok(_) => unreachable!(),
192/// Err(p_err) => {
193/// let data = p_err.get_ref();
194/// println!("recovered: {data}");
195/// }
196/// };
197/// ```
198/// [`Mutex`]: crate::sync::Mutex
199/// [`RwLock`]: crate::sync::RwLock
200#[stable(feature = "rust1", since = "1.0.0")]
201pub struct PoisonError<T> {
202 data: T,
203 #[cfg(not(panic = "unwind"))]
204 _never: !,
205}
206
207/// An enumeration of possible errors associated with a [`TryLockResult`] which
208/// can occur while trying to acquire a lock, from the [`try_lock`] method on a
209/// [`Mutex`] or the [`try_read`] and [`try_write`] methods on an [`RwLock`].
210///
211/// [`try_lock`]: crate::sync::Mutex::try_lock
212/// [`try_read`]: crate::sync::RwLock::try_read
213/// [`try_write`]: crate::sync::RwLock::try_write
214/// [`Mutex`]: crate::sync::Mutex
215/// [`RwLock`]: crate::sync::RwLock
216#[stable(feature = "rust1", since = "1.0.0")]
217pub enum TryLockError<T> {
218 /// The lock could not be acquired because another thread failed while holding
219 /// the lock.
220 #[stable(feature = "rust1", since = "1.0.0")]
221 Poisoned(#[stable(feature = "rust1", since = "1.0.0")] PoisonError<T>),
222 /// The lock could not be acquired at this time because the operation would
223 /// otherwise block.
224 #[stable(feature = "rust1", since = "1.0.0")]
225 WouldBlock,
226}
227
228/// A type alias for the result of a lock method which can be poisoned.
229///
230/// The [`Ok`] variant of this result indicates that the primitive was not
231/// poisoned, and the operation result is contained within. The [`Err`] variant indicates
232/// that the primitive was poisoned. Note that the [`Err`] variant *also* carries
233/// an associated value assigned by the lock method, and it can be acquired through the
234/// [`into_inner`] method. The semantics of the associated value depends on the corresponding
235/// lock method.
236///
237/// [`into_inner`]: PoisonError::into_inner
238#[stable(feature = "rust1", since = "1.0.0")]
239pub type LockResult<T> = Result<T, PoisonError<T>>;
240
241/// A type alias for the result of a nonblocking locking method.
242///
243/// For more information, see [`LockResult`]. A `TryLockResult` doesn't
244/// necessarily hold the associated guard in the [`Err`] type as the lock might not
245/// have been acquired for other reasons.
246#[stable(feature = "rust1", since = "1.0.0")]
247pub type TryLockResult<Guard> = Result<Guard, TryLockError<Guard>>;
248
249#[stable(feature = "rust1", since = "1.0.0")]
250impl<T> fmt::Debug for PoisonError<T> {
251 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
252 f.debug_struct(name:"PoisonError").finish_non_exhaustive()
253 }
254}
255
256#[stable(feature = "rust1", since = "1.0.0")]
257impl<T> fmt::Display for PoisonError<T> {
258 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
259 "poisoned lock: another task failed inside".fmt(f)
260 }
261}
262
263#[stable(feature = "rust1", since = "1.0.0")]
264impl<T> Error for PoisonError<T> {
265 #[allow(deprecated)]
266 fn description(&self) -> &str {
267 "poisoned lock: another task failed inside"
268 }
269}
270
271impl<T> PoisonError<T> {
272 /// Creates a `PoisonError`.
273 ///
274 /// This is generally created by methods like [`Mutex::lock`](crate::sync::Mutex::lock)
275 /// or [`RwLock::read`](crate::sync::RwLock::read).
276 ///
277 /// This method may panic if std was built with `panic="abort"`.
278 #[cfg(panic = "unwind")]
279 #[stable(feature = "sync_poison", since = "1.2.0")]
280 pub fn new(data: T) -> PoisonError<T> {
281 PoisonError { data }
282 }
283
284 /// Creates a `PoisonError`.
285 ///
286 /// This is generally created by methods like [`Mutex::lock`](crate::sync::Mutex::lock)
287 /// or [`RwLock::read`](crate::sync::RwLock::read).
288 ///
289 /// This method may panic if std was built with `panic="abort"`.
290 #[cfg(not(panic = "unwind"))]
291 #[stable(feature = "sync_poison", since = "1.2.0")]
292 #[track_caller]
293 pub fn new(_data: T) -> PoisonError<T> {
294 panic!("PoisonError created in a libstd built with panic=\"abort\"")
295 }
296
297 /// Consumes this error indicating that a lock is poisoned, returning the
298 /// associated data.
299 ///
300 /// # Examples
301 ///
302 /// ```
303 /// use std::collections::HashSet;
304 /// use std::sync::{Arc, Mutex};
305 /// use std::thread;
306 ///
307 /// let mutex = Arc::new(Mutex::new(HashSet::new()));
308 ///
309 /// // poison the mutex
310 /// let c_mutex = Arc::clone(&mutex);
311 /// let _ = thread::spawn(move || {
312 /// let mut data = c_mutex.lock().unwrap();
313 /// data.insert(10);
314 /// panic!();
315 /// }).join();
316 ///
317 /// let p_err = mutex.lock().unwrap_err();
318 /// let data = p_err.into_inner();
319 /// println!("recovered {} items", data.len());
320 /// ```
321 #[stable(feature = "sync_poison", since = "1.2.0")]
322 pub fn into_inner(self) -> T {
323 self.data
324 }
325
326 /// Reaches into this error indicating that a lock is poisoned, returning a
327 /// reference to the associated data.
328 #[stable(feature = "sync_poison", since = "1.2.0")]
329 pub fn get_ref(&self) -> &T {
330 &self.data
331 }
332
333 /// Reaches into this error indicating that a lock is poisoned, returning a
334 /// mutable reference to the associated data.
335 #[stable(feature = "sync_poison", since = "1.2.0")]
336 pub fn get_mut(&mut self) -> &mut T {
337 &mut self.data
338 }
339}
340
341#[stable(feature = "rust1", since = "1.0.0")]
342impl<T> From<PoisonError<T>> for TryLockError<T> {
343 fn from(err: PoisonError<T>) -> TryLockError<T> {
344 TryLockError::Poisoned(err)
345 }
346}
347
348#[stable(feature = "rust1", since = "1.0.0")]
349impl<T> fmt::Debug for TryLockError<T> {
350 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
351 match *self {
352 #[cfg(panic = "unwind")]
353 TryLockError::Poisoned(..) => "Poisoned(..)".fmt(f),
354 #[cfg(not(panic = "unwind"))]
355 TryLockError::Poisoned(ref p) => match p._never {},
356 TryLockError::WouldBlock => "WouldBlock".fmt(f),
357 }
358 }
359}
360
361#[stable(feature = "rust1", since = "1.0.0")]
362impl<T> fmt::Display for TryLockError<T> {
363 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
364 match&'static str *self {
365 #[cfg(panic = "unwind")]
366 TryLockError::Poisoned(..) => "poisoned lock: another task failed inside",
367 #[cfg(not(panic = "unwind"))]
368 TryLockError::Poisoned(ref p) => match p._never {},
369 TryLockError::WouldBlock => "try_lock failed because the operation would block",
370 }
371 .fmt(f)
372 }
373}
374
375#[stable(feature = "rust1", since = "1.0.0")]
376impl<T> Error for TryLockError<T> {
377 #[allow(deprecated, deprecated_in_future)]
378 fn description(&self) -> &str {
379 match *self {
380 #[cfg(panic = "unwind")]
381 TryLockError::Poisoned(ref p: &PoisonError) => p.description(),
382 #[cfg(not(panic = "unwind"))]
383 TryLockError::Poisoned(ref p) => match p._never {},
384 TryLockError::WouldBlock => "try_lock failed because the operation would block",
385 }
386 }
387
388 #[allow(deprecated)]
389 fn cause(&self) -> Option<&dyn Error> {
390 match *self {
391 #[cfg(panic = "unwind")]
392 TryLockError::Poisoned(ref p: &PoisonError) => Some(p),
393 #[cfg(not(panic = "unwind"))]
394 TryLockError::Poisoned(ref p) => match p._never {},
395 _ => None,
396 }
397 }
398}
399
400pub(crate) fn map_result<T, U, F>(result: LockResult<T>, f: F) -> LockResult<U>
401where
402 F: FnOnce(T) -> U,
403{
404 match result {
405 Ok(t: T) => Ok(f(t)),
406 #[cfg(panic = "unwind")]
407 Err(PoisonError { data: T }) => Err(PoisonError::new(data:f(data))),
408 }
409}
410